We calculate loop induced lepton flavor violating Higgs decays in the Littlest
Higgs model with T-parity. We find that a finite amplitude is obtained only when all
contributions from the T-odd lepton sector are included. This is in contrast to lepton fla-
vor violating processes mediated by gauge bosons where the partners of the right-handed
mirror leptons can be decoupled from the spectrum. These partners are necessary to can-
cel the divergence in the Higgs mass introduced by the mirror leptons but are otherwise
unnecessary and assumed to be decoupled in previous phenomenological studies. Further-
more, as we emphasize, including the partner leptons in the spectrum also introduces a
new source of lepton flavor violation via their couplings to the physical pseudo-Goldstone
electroweak triplet scalar. Although this extra source also affects lepton flavor changing
gauge transitions, it decouples from these amplitudes in the limit of heavy mass for the
partner leptons. We find that the corresponding Higgs branching ratio into taus and muons
can be as large as ~ 0.2 × 10 -6 for T-odd masses of the order a few TeV, a demanding
challenge even for the high luminosity LHC.

We discuss the two-photon coupling of the lightest scalar meson on the basis of an extension of ¿PT. Using low-energy data on the pion form factor and the ¿¿¿p+p-(p0p0) cross sections as inputs, we find G(s¿¿¿)¿0.126¿¿keV. The smallness of the result and the relative weight between its components, G¿¿¿S1G¿¿¿pp¿S1=1, suggests that the scalar 0++ meson is mainly a Q2Q¯2 state.

We discuss the two-photon coupling of the lightest scalar meson on the basis of an extension of χPT. Using low-energy data on the pion form factor and the γγ→π+π−(π0π0) cross sections as inputs, we find Γ(σ→γγ)≅0.126 keV. The smallness of the result and the relative weight between its components, Γγγ→S1Γγγ→ππ→S1≤1, suggests that the scalar 0++ meson is mainly a Q2Q¯2 state.

We worked out a generalization of su(2) chiral perturbation theory, including a perturbative singlet scalar field.
The approach suggests that the prediction for sensible low-energy observables converge faster towards their physical value. The physical mass and width of the scalar particle are obtained through a simultaneous analysis of the pion vector form factor and the ¿¿¿p
0
p
0 cross section. Both values are statistically consistent with the ones
obtained by using Roy equations in p-p scattering. In addition we find indications that the photon-photon-singlet
coupling is quite small.

We discuss forward-backward charge asymmetries for lepton-pair production in association with a large-transverse-momentum jet at hadron colliders. The lepton charge asymmetry relative to the jet direction AjFB gives a new determination of the effective weak mixing angle sin2lept
eff M2 Z with a statistical precision after cuts of 103 (8 103) at LHC (Tevatron). This is to be compared with the current uncertainty at LEP and SLD from the asymmetries alone, 2 104. The identification of b jets
also allows for the measurement of the bottom-quark–Z asymmetry AbFB at hadron colliders, the resulting statistical precision for sin2lept eff M2
Z being 9 104 (2 102 at Tevatron), also lower than the reported precision at e+e- colliders, 3 10-4.

We discuss forward-backward charge asymmetries for lepton-pair production in association with a large-transverse-momentum jet at hadron colliders. The lepton charge asymmetry relative to the jet direction AjFB gives a new determination of the effective weak mixing angle sin2lept
eff M2 Z with a statistical precision after cuts of 10 3 (8 10 3) at LHC (Tevatron). This is to be compared with the current uncertainty at LEP and SLD from the asymmetries alone, 2 10 4. The identification of b jets
also allows for the measurement of the bottom-quark–Z asymmetry AbFB at hadron colliders, the resulting statistical precision for sin2lept eff M2
Z being 9 10 4 (2 10 2 at Tevatron), also lower than the reported precision at e+ e- colliders, 3 10 -4.

The amplitude for the anomalous transitions gamma pi(+) -> pi(0) pi(+) is analyzed within Chiral Perturbation Theory including electromagnetic interactions. The presence of a t-channel one-photon exchange contribution induces sizeable e^2 corrections which enhance the cross-section in the threshold region and bring the theoretical prediction into agreement with available data. In the case of the crossed reaction gamma pi(0) -> pi(+) pi(-), the same contribution appears in the s-channel and its effects are small.